Many components made of polymer materials are increasingly being used in the construction of motor vehicles: such as dashboards, door panels, consoles, etc. Some of them are used very visibly in the internal architecture of the vehicle, in such a way that they must have, on the visible surface, a pleasing aesthetic appearance which reproduces, for example, the appearance of leather. This visible surface constitutes the skin of the component. This skin, bulk coloured, must also have other properties, which are good scratch resistance and good resistance to chemicals such as solvents, as well as the ability to withstand thermal variations, ranging from the very cold to the prolonged exposure to sunshine in a confined atmosphere.
The abovementioned skin may concern all components (also called inserts) made of polymer materials, particularly those involved in the internal architecture of the vehicle--from the rigid inserts generally reserved for bottom-of-the-range vehicles to the foamed inserts with which more sophisticated vehicles are equipped.
In the case of rigid inserts, the skin, or at very least its appearance, is obtained by the etching of the mould, either as a result of the constituent material of the insert or else, for example, by the co-injection moulding of two different materials, one for the insert and the other for the skin.
In the case of foamed inserts, the skin is generally made, independently of the insert itself, of a material compatible, on the one hand, with the foam subsequently developed between the said skin and the insert and, on the other hand, with the finishing lacquers, such as polyurethane lacquers, deposited on the visible surface of the said skin in order to make them comply, where necessary, with the specifications of motor-vehicle manufacturers.
In order to produce the skin of inserts, such as the dashboard for example, various processes have been proposed, one of the preoccupations of which is to produce a skin with the lowest possible residual stress.
In a first type of process which uses a premanufactured polypropylene-based sheet, the skins of dashboards are produced by means of the process of positive or negative thermoforming. However, the thermo-forming employed, whether positive (the surface appearance of the skin exists on the sheet before thermoforming) or negative (the surface appearance is given to the sheet by the mould during thermoforming), generally involves, both for technical and economic reasons, sheets (preheated) which are preferably thin. Consequently, the shapes of the skins obtained are very limited and contain residual stresses which, when they are relieved by ageing, generate crazes which are very unattractive.
In one type of process, the skins of dashboards are produced using the slush moulding process (mentioned above) which generally allows them to be obtained free of residual stresses. By itself, the slush moulding process is a moulding process using conventional equipment (electroformed nickel mould heated by a hot-air system) which allows the production of the desired skins using a polymer powder based on polyvinyl chloride (PVC) by the technique of powder slush moulding. The skins thus produced, although they no longer include residual stresses, or very few residual stresses, and although they meet for the most part the specifications of motor-vehicle manufacturers, do have immediate or potential drawbacks, even in the short term.
Among the immediate drawbacks, the PVC-based skins are quite highly loaded, owing to their composition, with volatile substances (in particular, PVC plasticizers). These materials, in use and due to the effect of temperature variations in the passenger compartment of the vehicle, are volatilized and migrate to cold areas, where they condense--this is the well-known phenomenon of slow opacification of the windscreen of a vehicle, which is a particular impediment to the driver's view and to the proper control of the vehicle.
Among the potential drawbacks, the use of PVC appears, in the current situation of end-of-life recycling of materials used in the manufacture of motor vehicles, to be prohibited in the relatively short term for better environmental protection.
Polymer materials resulting from grinding up worn-out vehicles are currently used (completely or partly) as fuel in certain types of furnaces, such as, for example, furnaces in cement works. However, PVC converted into a fuel material emits, when it is burnt, acid-gas effluents which are harmful to the environment.
This is why, in order to meet the twin concern of motor-vehicle manufacturers and the public authorities, which is firstly that of protecting the environment (removing harmful gaseous effluents generated by the combustion of waste made of polymer materials and limiting the dumping thereof) and then that of more completely recycling the waste (formed from polymer materials) resulting from the destruction of worn-out vehicles by grinding, motor-vehicle manufacturers seriously envisage limiting the number of polymers present in each vehicle, but at the same time increasing their relative amounts, and in particular substituting PVC with polyolefins in order to allow easier recycling. Thus, motor vehicles would be equipped with components, such as foamed dashboards, whose skin, foam and rigid insert would be made of polyolefins which can be recycled at the end of the life of the vehicles.
To do this, polyolefin thermoplastic compositions have already been proposed.
By way of example, document EP 0,508,801 describes a powder composition of thermoplastic elastomers for powder moulding, which comprises:
either (i) a powder of a thermoplastic elastomer comprising a composition composed of an ethylene-alpha-olefin copolymer rubber and of a polyolefin resin; PA1 or (ii) a powder of a thermoplastic elastomer comprising a partially crosslinked composition composed of an ethylene-alpha-olefin copolymer rubber and of a polyolefin resin, PA1 in order to increase melt flow and at the same time to improve the flexibility of the moulded article, the olefin copolymer rubber is diluted with oil and thus comprises an ethylene-alpha-olefin copolymer rubber with the addition of a mineral oil as a flexibilizer with a relatively high content with respect to the polyolefin elastomer. These oils constitute, in fact, a risk of emitting volatile substances which pollute the environment and opacify vehicle windscreens, and also a risk of impairing the appearance of the skin during its ageing--the phenomenon of exudation is not eliminated. PA1 finally, in order to allow the skin to be easily demoulded, external mould-release agents (for example, dimethylsiloxane) deposited on the mould or mould-release agents internal to the composition. PA1 (for example, methylpolysiloxane) are employed: they also constitute a risk of directly polluting the environment of the manufacturing workshops, but above all they make it more difficult to carry out the subsequent operation of lacquering the skins, this currently being necessary in order to make them comply with the specifications of motor-vehicle manufacturers (appearance, matt-gloss level, scratch resistance, abrasion resistance and resistance to chemicals, including solvents). PA1 the first being polypropylene (isotactic index of between 95 and 98) or a copolymer of propylene and of ethylene and/or of an alpha-olefin of the CH2.dbd.CHR type where R is an alkyl radical having from 2 to 6 carbon atoms, containing more than 85% by weight of propylene and having an isotactic index of greater than 85; PA1 the second being a fraction of an ethylene-containing polymer (insoluble in xylene at room temperature); PA1 the third being a fraction of an amorphous ethylene-propylene copolymer containing from 40 to 70% by weight of ethylene (soluble in xylene at room temperature). PA1 the first (10 to 40 parts by weight) is a propylene homopolymer or a copolymer of propylene with ethylene and/or a C4 to C10 alpha-olefin; PA1 the second (0 to 20 parts by weight) of a copolymer of ethylene and of propylene and/or a C4 to C10 alpha-olefin; PA1 the third (50 to 80 parts by weight) of an elastomeric copolymer of ethylene and of propylene and/or a C4 to C10 alpha-olefin and optionally with a minor quantity of a diene. PA1 either directly by polymerization, controlled by agents for regulating the molecular weight; PA1 or by subjecting the polymer composition to a "visbreaking" action which consists in cutting the molecular chains present by means of a peroxide. PA1 is in the form of a readily pourable fine powder, to be used in the slush moulding process; PA1 has a substantially improved melting behaviour when it is in contact with the hot mould; PA1 when being used, provides the skins with all the characteristics required by the functional specifications of motor-vehicle manufacturers, in particular good thermal, scratch and abrasion resistance and good resistance to chemicals including solvents; PA1 allows the formation of skins having no residual stresses; PA1 is compatible with the subsequent lacquering operations, should this lacquering be necessary; PA1 does not contain agents having high vapour pressures; PA1 does not manifest whitening phenomena at reverse tapers during demoulding or during subsequent handling of the skin; PA1 allows excellent demoulding of the component formed; PA1 can be completely recycled, after use, within the motor-vehicle product line in order to meet the environmental protection requirements; and PA1 meets the technical and economic requirements. PA1 a) from 97 to approximately 5 parts by weight and preferably from 95 to 10 parts by weight of a matrix composed of at least one polymer containing at least 50 mol % of propylene and of at least one at least partially crosslinked polyolefin elastomer; PA1 b) from 3 to approximately 95 parts by weight and preferably from 5 to 90 parts by weight of at least one polymer containing at least 50 mol % of ethylene; and PA1 c) from 0 to 6 parts by weight of an internal mould-release agent per 100 parts by weight of components "a" and "b". PA1 used alone, such as: ethylene-propylene rubber (EPR), ethylene-propylene-diene monomer (EPDM), styrene-butadiene-styrene (SBS), styrene-ethylene-butadiene-styrene (SEBS), styrene-butadiene rubber (SBR), this being at least partially crosslinked; PA1 or used as a blend, either in a reactor or by extrusion, of polypropylene with at least one elastomer, such as, for example, polypropylene/ethylene-propylene rubber (PP/EPR), polypropylene/ethylene-propylene-diene monomer (PP/EPDM), the elastomeric fraction of which is at least partially crosslinked. PA1 from 5 to 95% and preferably from 80 to 20% by weight of at least one polymer containing at least 50 mol % of propylene; and PA1 from 95 to 5% and preferably from 20 to 80% by weight of at least one at least partially crosslinked polyolefin elastomer. PA1 magnesium stearate [Mg(C18H3502)2]: fine powder PA1 it easily replaces, by itself, the pair consisting of calcium stearate/ethyl-bis amide in which the ethyl-bis amide, which reduces the melt viscosity of the polymer composition containing them, promotes, excessively, the migration of the calcium stearate to the mould/material interface; PA1 it is used, for the same reason as calcium stearate, as an antioxidant for polypropylenes by limiting, or even preventing, thermo-oxidative degradation of the polymer at the surface, this being the cause of a reduction in the scratch, abrasion and solvent resistance properties; PA1 its presence ensures excellent demoulding of the skin; and PA1 it allows the skin to be lacquered with good adhesion of the lacquer after a special preparation, such as flame brushing or the use of an adhesion primer. PA1 in mixing the constituents of the composition in an extrusion zone; and PA1 in cryogenically grinding the granules coming from the extrusion, in such a way that this results in a thermoplastic polyolefin powder having elastic properties. PA1 white, loose, hydrophobic powder PA1 BET specific surface area (DIN 66131 standard): 90 m2/g PA1 average diameter of the agglomerates: 5 .mu.gm PA1 compacted density (DIN-ISO 787/XI standard): 100 g/l.
this powder composition being able to be used in the slush moulding process.
However, according to this document:
Another document, U.S. Pat. No. 5,302,454, also describes a thermoplastic polyolefin composition, having elastic properties utilizable in the automotive field, which is in a pulverulent form.
This composition includes three components:
However, such a composition appears not to be able, except by making certain modifications by adding special agents such as, for example, polyolefin resins, to be easily processed in the slush moulding process (risk of clumping and difficulty in demoulding).
Finally, another document, European Patent Application published under the number EP 0,637,610, describes a process and a thermoplastic polyolefin composition used in slush moulding for the manufacture of articles (laminating) which is in pulverulent form.
This composition includes three fractions:
According to this document, the desired intrinsic viscosity of the polymer composition used in the process in question may be obtained:
Such a polymer composition has the same drawbacks as those mentioned previously.
Consequently, the problem of using polyolefin compositions as substitutes for polyvinyl chloride in the production of skins for the foamed or non-foamed covering of the inserts remains almost in its entirety.
Thus, the objectives of the invention are to provide a polyolefin composition having elastic properties, for the production of skins, which: